Optimize `prereqs` Dockerfile to checkout Git submodules first
Optimize `prereqs` Dockerfile to checkout Git submodules first

Due to Docker build cache, this can usually save me 20s+ build time on my MacbookPro each time i
rebuild after a local code change.

grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web